Project Summary/Abstract The death of sensory hair cells in the inner ear causes irreversible hearing loss and balance deficits in humans and other mammals. In non-mammalian vertebrates, hair cell death elicits a regenerative response in neighboring supporting cells which can include both proliferation and differentiation into replacement hair cells. These repair processes are observed in mammals at early developmental stages, yet decline during maturation (e.g. the first postnatal week in mice). The cause of this reduction in supporting cell regenerative responses remains undefined. A better understanding of the signals controlling supporting cell proliferation will aid efforts to identify the limitations of hair cell regeneration in mammals, and ultimately could lead to therapies that restore sensory function. Supporting cell shape changes are an early event in the regenerative response associated with proliferation (Cotanche 1987; Meyers et al. 2007), and mechanical signals are now recognized as a key mediator of cell behavior. This leads to the hypothesis that mechanical signals arising from cell loss stimulate regenerative proliferation in supporting cells. This project has three aims to test hypothesized ?triggers? and ?brakes? of sensory hair cell regeneration related to mechanical signaling. The first investigates mechanosensory pathways with well-established roles in tissue regeneration that remain relatively unexplored in the inner ear. The second aim tests whether the signals that initiate proliferation are specific to the loss of hair cells, or rather are more general cues related to loss of other cell types in the sensory epithelium. The final aim is to identify proteins that impart unique thickness and stability to the apical F-actin belts of mature mammalian supporting cells, which are hypothesized to limit hair cell regeneration in mammals.